Embodiments of the invention relate generally to magnetic resonance imaging (MRI) systems and, more particularly, to the monitoring and treating of a coolant in a cooling system of a magnetic resonance imaging (MRI) system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B1), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Thermal management of the gradient subsystem, as well as other elements in an MRI system, becomes more challenging with demands for increased power, speed and duty cycle. For example, with increased power, speed, and duty cycle in the MRI system comes an increase in the amount of current and voltage supplied by the gradient drivers to the gradient coil. This increase in voltage and current results in increased heat in the system. Thus, many MRI systems employ a cooling system (i.e., heat exchanger) that cools heat-generating components in the gradient sub-system by pumping a coolant through hollow copper tubing attached to or positioned near the heat generating components in the gradient driver and the gradient coils. Cooling tubes may also be attached to other components in the resonance assembly of the MRI system in proximity to the gradient driver and gradient coils. Typically, water is used as the coolant because of its high specific heat capacity and a high thermal conductivity.
In addition to generating increased amounts of heat, the increased voltage and current applied to the gradient drivers also results in a transfer of a leakage current from the electrical components to the coolant. The leakage current is accounted for by maintaining a minimum coolant electrical resistivity to prevent a stray current in the MRI system. To control the electrical resistivity of the coolant, deionized water can be used as the coolant, as deionized water has an extremely high electrical resistivity. However, there are several drawbacks and limitations to the use of deionized water as a coolant. First, deionized water is corrosive to the copper tubing of the cooling system, which can shorten the life span of the cooling system and affect the performance of the MRI system. Second, the electrical resistivity of the deionized water lowers over time, becoming ionized after extensive usage of the MRI system. Thus, a desired minimum coolant electrical resistivity may not be achievable after prolonged usage of the MRI system.
It would therefore be desirable to have a system and method capable of maintaining a desired minimum electrical resistivity in the coolant circulated through a cooling system of an MRI system. It would further be desirable to have a system and method capable of providing a coolant having a minimal corrosive effect on the components of the cooling system.